1. Field of the Invention
The present invention relates to system for controlling an engine equipped with a fuel injector through which fuel in injected directly into an engine.
2. Description of the Related Art
Typically, three-way catalysts are known as a catalytic operative to simultaneously and quite effectively lowers emission levels of HC, CO and NOx in exhaust gas from an internal combustion engine so as thereby to purify the exhaust gas at an approximately stoichiometric air-to-fuel ratio. However, a diesel engine is operated with an air-to-fuel ratio in an extremely lean state (for example, A/Fxe2x89xa718), so the three-way catalyst can not reduce NOx in exhaust gas. Furthermore, when the air-to-fuel ratio is in the extremely lean state, the oxygen concentration of exhaust gas becomes extremely high, so that it is difficult to reduce NOx sufficiently in such an atmosphere even through a NOx purifying catalyst. In this regards, the same thing can be said of a gasoline engine with air-to-fuel ratios in a lean range. To the contrary, there has been known a technology in which utilization is made of a so-called NOx trapping substance that absorbs NOx in an excess oxygen exhaust gas with an oxygen concentration higher than a predetermined oxygen concentration of, for example, 4%, and releases the absorbed NOx as the oxygen concentration drops. However, it is necessary for the NOx trapping substance to carry out so-called refreshing that is referred to releasing the absorbed NOx before the absorption capacity declines, since the NOx trapping substance encounters a decline in absorption capacity with an increase in the amount of NOx absorption.
A fuel injection system such as disclosed in, for example, Japanese Unexamined Patent Publication No. 6-200045, performs primary fuel injection ordinarily at a point of time near a top dead center of a compression stroke and, however, performs post fuel injection at a point of time between an expansion and an exhaust stroke in addition to the primary fuel injection at the point of time near a top dead center of a compression stroke so as thereby to enrich an exhaust gas or decreasing an air-to-fuel ratio of the exhaust gas (which is referred to as an exhaust gas air-to-fuel ratio) in order to release NOx from the NOx trapping substance disposed in an exhaust passage. NOx released from the NOx trapping substance reacts to HC and/or CO in the exhaust gas to be deoxidized. The fuel injection control means further discloses that, in order to prevent a large amount of unburned fuel from being admitted into an intake air stream, an exhaust gas recirculation passage is shut off when the fuel injection control for refreshing the NOx trapping substance in an engine operating region of moderate and lower engine loads where an increase in the amount of exhaust gas is required.
Japanese Unexamined Patent Publication No. 10-252543 discloses improving the NOx conversion efficiency of a NOx deoxidization catalyst disposed in an exhaust passage by changing a post-fuel injection timing so as to reform highly fuel that is sprayed through post-fuel injection according to in-cylinder temperatures that are estimated on the basis of a temperature of then exhaust gas.
When the post-fuel injection is implemented, a large amount of NOx is released abruptly at the beginning of a change in exhaust gas air-to-fuel ratio from a lean state to a rich state (including an exhaust gas air-to-fuel ratio represented by an excess air ratio (xcex) of approximately 1 (one), so that there occurs a lack of HC for NOx deoxidization, as a result of which the NOx trapping substance encounters aggravation of NOx conversion efficiency. In order to provide an amount of HC sufficient to deoxidize the temporarily increased amount of NOx released from the NOx trapping substance, though it can be acceptable to increase the amount of post-fuel injection so as to provide highly reformed fuel by advancing a post-fuel injection timing, nevertheless, it causes aggravation of fuel consumption.
Diesel engines are operated with significantly low air-to-fuel ratios and, in consequence, and, in consequence, often generate exhaust gases at low temperatures below than 200xc2x0 C. When the exhaust gas is at a low temperature like this, even though the post-fuel injection is implemented to raise the air-to-fuel ratio so as thereby to produce an atmosphere suitable for the NOx deoxidization catalyst to deoxidize NOx or to cause the NOx trapping substance to release NOx, it is difficult to deoxidize NOx and purify the exhaust gas because of low activity of the catalyst itself. When the NOx trapping substance is at lower temperatures, it is hard to release NOx as expected even when an exhaust gas air-to-fuel ratio is raised. Further, though an exhaust gas temperature raises due to an advance in post-fuel injection timing, nevertheless, the fuel sprayed through the post-fuel injection becomes apt to burn in the cylinder, so as to cause a less increase in the amount of HC in the exhaust gag. In this regard, it is unfavorable for the NOx trapping substance to advance the post-fuel injection timing.
It is an object of the present invention to provide a control system for controlling an engine which prevents aggravation of NOx conversion efficiency of an NOx trapping substance at the beginning of a change in exhaust gas air-to-fuel ratio from a lean state to a rich state due to post-fuel injection that is implemented to release NOx from a NOx trapping substance.
It is another object of the present invention to provide a control system for controlling an engine which activates early a catalyst while post-fuel injection is implemented.
The foregoing objects of the present invention have been accomplished by an engine control system devised on the basis of the fact, which has been revealed by the inventors of the present application, that a relatively large increase in the amount of HC is caused in an exhaust gas by changing a post-fuel injection timing without increasing the amount of post-fuel injection,
According to an aspect of the present invention, a control system for an engine equipped with a fuel injector through which fuel is sprayed directly into a combustion chamber of the engine and a catalyst containing a NOx trapping substance which is operative to absorb NOx in an excess oxygen exhaust gas and release NOx into the exhaust gas when there is a drop in oxygen concentration and, as a result of which, a reaction of HC to NOx released from the NOx trapping substance so as thereby to purify the exhaust gas, comprises fuel injection control means for performing fuel injection through primary fuel injection which is made at a primary fuel injection timing near a dead top center of a compression stroke and post-fuel injection which is made at a post-fuel injection timing between an expansion stroke and an exhaust stroke after the primary fuel injection when causing a drop in oxygen concentration so as to release NOx from the NOx trapping substance, wherein the control means controls a retardation of the post-fuel injection timing so as to be large for a specified period of time from commencement of the post-fuel injection and to be small after a lapse of the specified period of time.
The term xe2x80x9cexcess oxygen atmospherexe2x80x9d or xe2x80x9cexcess oxygen exhaust gasxe2x80x9d as used herein shall mean and refer to atmosphere or exhaust gas in a state where oxygen exists sufficiently around a NOx trapping substance so much as to cause the NOx trapping substance to absorb NOx in an exhaust gas and prevent the NOx trapping substance from releasing NOx into the exhaust gas. Specifically, the excess oxygen atmosphere or excess oxygen exhaust gas has an oxygen concentration higher than a specific value of, for example, 4% and an exhaust gas air-to-fuel ratio is higher than a stoichiometric air-to-fuel ratio, for example equal to or greater than 18. The exhaust gas air-to-fuel ratio is consistent with an air-to-fuel ratio of an air-fuel mixture in a combustion chamber (which is referred to as a combustion chamber air-to-fuel ratio and shall mean a mean air-fuel ratio of an air-fuel mixture in a combustion chamber) in the case where fuel and/or secondary air are not supplied directly into the exhaust passage. Further, a drop in oxygen concentration may be to a value greater than 3% but less than 4%, more desirably greater than 1% but less than 2%. For example, attainment of a rich state where an exhaust gas air-to-fuel ratio reaches a stoichiometric air-to-fuel ratio or a value near the stoichiometric air-to-fuel ratio fulfills this xe2x80x9cdrop in oxygen concentrationxe2x80x9d.
With the engine control system thus constructed, while an exhaust gas is in a lean state, NOx in the exhaust gas is absorbed by the NOx trapping substance and prevented from being emitted into the atmosphere. When the NOx should be released from the NOx trapping substance, the amount of intake air or fuel injection admitted into the engine is controlled to change a combustion chamber air-to-fuel ratio toward a rich side so as thereby to cause a drop in oxygen concentration of exhaust gas around the NOx trapping substance and, concurrently, fuel injection is implemented through primary fuel injection which is made at a point of time near a top dead center of a compression stroke and post-fuel injection which is made at a point of time in an expansion and an exhaust stroke after implementation of the primary fuel injection. A change in mean combustion chamber air-to-fuel ratio causes restraint of NOx generation and a drop in oxygen concentration of exhaust gas as well. Furthermore, implementation of the post-fuel injection produces an increased amounts of HC (unburned HC and partly deoxidized and reformed HC) and CO, oxygen in the exhaust gas is consumed for deoxidization of HC and CO on the catalyst. As a result, the oxygen concentration around the NOx trapping substance drops, NOx that is absorbed in the form of NO3xe2x88x92 in the NOx trapping substance is released in the form of NO2. Since the release of NOx occurs relatively rapidly due to commencement of the post-fuel injection, a NOx concentration raises sharply around the NOx trapping substance. To the contrary, with the engine control system of the present invention, since a retardation of post-fuel injection timing from a top dead center of a compression stroke is made large for a period of time from commencement of the post-fuel injection, the large retardation restraints combustion of the fuel sprayed through the post-fuel injection an effect of causing an increase in HC generation as compared with a small retardation. Accordingly, although NOx is released abruptly, a deoxidization material, i.e. HC, necessary to deoxidize NOx is prevented from being in short supply. Further, though the amount of NOx that is released from the NOx trapping substance decreases sharply with a lapse of time, since the retardation of post-fuel injection timing is changed smaller after a lapse of a specified period of time, the control prevents an exhaust gas from raising an level of HC in excess therein and emitting the HC into the atmosphere as it is.
The retardation of post-fuel injection timing that is made for the specified period of time is desirable to be greater than 30xc2x0 in crank angle but less than 50xc2x0 in crank angle (before an early half of an expansion stroke). This is because a retardation of post-fuel injection timing greater than 30xc2x0 in crank angle from a top dead center of a compression stroke yields an effect of significantly increasing the amount of HC in the exhaust gas, and an excessive retardation of post-fuel injection timing causes a drop in in-cylinder temperature, and hence exhaust gas temperature, which leads to aggravation of engine combustibility, aggravation of releasing performance of the NOx trapping substance and poor deoxidization of NOx.
The post-fuel injection timing after a lapse of the specified period of time may be set so as to bring a concentration ratio of a CO concentration relative to a HC concentration of an exhaust gas becomes greater than a specified value. This is because, when CO concentrates around the NOx trapping substance due to an increase in the amount of CO generation, NOx in the NOx trapping substance becomes apt to secede from the NOx trapping substance due to replacement with CO (at this time the NOx trapping substance is transformed into a form of carbonate). In consequence, refreshing the NOx trapping substance is achieved quickly, so that the period of time for which the post-fuel injection is made is shortened with an effect of preventing or significantly reducing aggravation of fuel consumption.
In the case where the is equipped with an exhaust gas recirculation system for admitting partly an exhaust gas into an intake air stream, the post-fuel injection timing after a lapse of the specified period of time from commencement of the post-fuel injection may be set so as to lower a HC concentration of the exhaust gas while the exhaust gas is recirculated. As described above, it is effective to increase an HC concentration of an exhaust gas by implementing the post-fuel injection in order to cause the NOx trapping substance to release NOx and purify the exhaust gas, which, however, results in supply of an increased amount of unburned fuel into an intake air stream while the exhaust gas is recirculated, so as to cause aggravation of fuel consumption. In particular, in the case where the amount of exhaust gas recirculation is increased as measures to prevent smoke generation when a mean combustion chamber air-to-fuel ratio changes onto a rich side due to implementation of the post-fuel injection, it is a problem that a large amount of unburned HC is admitted into an intake air stream. Therefore, in the engine control system of the present invention, since the NOx trapping substance requires a less amount of HC after a progress of releasing NOx from the NOx trapping substance, the retardation of post-fuel injection timing is changed smaller so as to cause a decrease in the amount of HC generation in the exhaust gas, thereby preventing unburned HC from being admitted into the intake air stream. In this case, the retardation may be less than 20xc2x0 in crank angle from a dead center from a compression stroke.
Though it is desirably to utilize barium as the NOx trapping substance, utilization may be made of one selected from a group of other alkaline earth metals, alkaline metals and rear earth metals or a combination of two selected from the group. The NO trapping substance may be used in combination with NOx purifying catalyst or supported together with a NOx purifying catalytic metal on a catalyst substrate. Otherwise, the NOx trapping substrate and a NOx purifying catalytic metal may be layered on a honeycomb substrate and disposed in an exhaust passage such that the NOx trapping substance layer is upstream from the NOx purifying metal layer with respect to an exhaust gas stream. It is also acceptable to use the NOx trapping substance in combination with a three-way catalyst. The NOx purifying catalyst may be of a type supporting a noble metal such as platinum or a transition metal on zeolite or the like.
According to another aspect of the present invention, the control system for an engine equipped with a fuel injector through which fuel is sprayed directly into a combustion chamber of the engine and a catalyst disposed in an exhaust passage and operative to oxidize HC performs fuel injection control in which primary fuel injection is made at a primary fuel injection timing near a dead top center of a compression stroke and post-fuel injection is made at a post-fuel injection timing between an expansion stroke and an exhaust stroke after implementation of the primary fuel injection. The fuel injection control system comprises temperature detection means for detecting a temperature of the catalyst, and control means for making a retardation of the post-fuel injection timing large for a specified period of time from commencement of the post-fuel injection and, on the other hand, making it small after a lapse of the specified period of time when the temperature of the catalyst is lower than a specified temperature.
With the engine control system, in conditions where an exhaust gas is at low temperatures, though the catalyst is low in activity and, in consequence, performs less sufficient purification of the exhaust gas, there is provided an increase in the amount of HC in the exhaust gas by implementing the post-fuel injection at a timing temporarily retarded greatly, so that an oxidizing reaction of HC becomes apt to progress and, in consequence, activation of the catalyst is promoted due to a raise in catalyst temperature which is caused by reaction heat. Since catalyzing reaction progresses well with a high efficiency after the catalyst has gained sufficient activity once, the catalyst activity is kept even when the retardation of post-fuel injection timing is made small, so as to purify the exhaust gas efficiently. Further, making the retardation of post-fuel injection timing small improves combustibility of the fuel sprayed through the post-fuel injection with an effect of preventing aggravation of an emission level of NOx due to a state where an excess of HC continues.
Utilization may be made of a NOx purifying catalyst as the catalyst which is effective in deoxidizing HC. In this case, the catalyst is improved in NOx purification efficiency and, when used in combination with a NOx trapping substance, purifies efficiently NOx that is released from the NOx trapping substance.
As described above, the engine control system which performs the primary fuel injection which is made at a point of time near a top dead center of a compression stroke and the post-fuel injection at a point of time after the primary fuel injection when causing the NOx trapping substance to release NOx and makes a retardation of post-fuel injection timing from the top dead center of a compression stroke for a specified period of time from commencement of the post fuel injection large and, however, small after a lapse of the specified period of time, enables it to supply a large amount of HC around the NOx trapping substance when causing the NOx trapping substance to release NOx quickly with an effect of preventing an occurrence of a lack of HC for deoxidization pf the NOx that is released from the NOx trapping substance, which is always advantageous for improvement of NOx conversion efficiency. Moreover, supply of a large amount of HC causes instantaneously a raise in temperature of a noble metal of the catalyst, as a result of which the catalyst enhances improvement of NOx conversion efficiency.
Further, the engine control system, when implementing the primary fuel injection which is made at a point of time near a top dead center of a compression stroke and the post-fuel injection at a point of time between an expansion and an exhaust stroke after implementation of the primary fuel injection, makes a retardation of post-fuel injection timing from the top dead center of a compression stroke for a specified period of time from commencement of the post fuel injection large and, however, small after a lapse of the specified period of time, while the catalyst is at temperatures lower than a specified value. Accordingly, it is enabled to activate the catalyst quickly by temporarily supply a large amount of HC to the catalyst, as a result of which catalytic conversion efficiency of the catalyst is improved even at lower exhaust gas temperatures.